1. Field of the Invention
The present invention relates to high melt strength polymers. In particular, the present invention concerns linear poly-propylene having high melt strength and comprising a high molecular weight portion and a low or medium molecular weight portion. The invention further concerns a process for preparing linear homopolymers and copolymers of propylene having high melt strength. Finally, the present invention relates to the use of the new polypropylene for the manufacture of products by, e.g., foaming, melt-stage thermo-forming, blow moulding, extrusion coating, fibre forming and injection moulding.
2. Description of Related Art
An important characteristic of polypropylene (PP) homopolymers is their stiffness, which is closely related to the crystallinity and the molecular structure of the polymer chain. Further, PP homopolymers have good resistance to deformation at elevated temperatures, high tensile strength, surface hardness, and good toughness at ambient temperatures. By copolymerizing propylene with other monomers it is possible further to improve the impact resistance.
The thermoformability of polypropylene in melt phase is, however, rather poor, which can be mentioned as a problem with polypropylene. This feature is primarily due to sheet sag during heating and forming processes. Generally, a material which is thermo-formable in the melt-phase should possess a viscous component to allow for flow when stressed and a sufficiently elastic component to resist flow when not stressed. Another important factor affecting thermoforming is the melt drawability of the polymer which depends on extensional viscosity. Similarly, the viscous and elastic components are beneficial for forming of blow moulded and foamed PP products.
The two major parameters that contribute to the viscoelastic behaviour are molar mass and molar mass distribution.
Melt strength means the force needed in extension to break the polymer melt. Linear polymers generally have considerably low melt strength. For polypropylene the most determining parameter is the molar mass: the higher the molecular mass of the material, the higher the melt strength.
Known in the art (WO 94/26794, PoliBrasil) is a method of preparing high melt strength polypropylene by a continuous process, wherein propylene is polymerized in at least two polymerization vessels by adding discrete amounts of hydrogen as a molar mass regulator to at least one of the polymerization vessels to produce high melt strength polypropylene. The resulting polymer product contains 10 to 35% of a higher molar mass portion and 65 to 90% of a lower molar mass portion. The process equipment comprises five slurry reactors connected in series, the amount of hydrogen fed to the polymerization vessels being successively reduced from the second polymerization vessel on.
In rheological terms, the polypropylene of WO 94/26794 is unsatisfactory as a high melt strength resin, because the zero viscosity and the shear thinning behaviour are rather similar to that of a conventional polypropylene used for comparison in the Examples of the reference. Thus, the products are not well suited to the manufacture of, e.g., foamed articles. Very little information on the catalysts used is given; in particular there is no teaching on hydrogen sensitivity of the catalyst or on any catalyst donors employed.
Further, because of the afore-mentioned process configuration, no hydrogen can be removed between the reactors, which limits the possibilities of varying the process conditions of the polymerization reactors. Thus, e.g., the high molar mass portion has to be made in the first reactor and the low molar mass portion in the second reactor. In a slurry process (such as the one of WO 94/26794), carried out e.g. in heptane slurry, generally a TiCl3 catalyst having low activity is used, because the activity level has to be controlled in the process due to limited heat transfer in these kind of processes.
Another patent, EP 0 498 603 (Amoco), describes a multireactor process for preparing polypropylene having a broad molecular mass distribution which, expressed as the ratio Mw/Mn, lies in the range of 7 to 11. This high stiffness polypropylene, which has high shear thinning, is suitable for, e.g. moulding thin walled products. The manufacture of foamed articles is not mentioned and the propylene polymers of the reference do not exhibit the necessary melt strength for the production of light-weight polymer foams with closed cells.
Still another prior art publication, EP 0 573 862 (Himont), describes a polypropylene of very broad molecular mass distribution, Mw/Mn>20. The polypropylene is claimed to have improved mechanical properties of high stiffness and improved resistance due to the broad MWD. The prior art process proposed for preparing the product comprises a multi-reactor gas phase process. The products are suggested for extrusion in thin sheets to be subjected to thermoforming. Once again, there is no suggestion of any use in foamed articles with closed cells and the polymers would not appear to have sufficiently high melt strength.